1. Stratospheric Ozone: Production, Destruction, & Trends
Antarctic Ozone Hole:
Sept. 12, 2012

2. Take away concepts Why is the Ozone layer important?
Atmospheric thermal structure
UV radiation and stratospheric O2 and O3.
Natural ozone production and destruction
The life cycle of CFCs, and their role in ozone destruction.
Why the Antarctic ozone hole is larger than the arctic, and why there is no "hole" over the tropics.
How large is the ozone hole now?
Future trends

3. Ozone (O3) Triatomic oxygen molecule
Max concentration is ~10 ppm, occurring in Stratosphere (15-30 km).
Stratosphere contains 90% of all ozone.
Tropospheric ozone from auto emissions (bad)

4. What does the ozone layer do for us?
Ozone absorbs UV radiation (between 210 and 290 nm).
Radiation at these wavelengths harm biomolecules (DNA)
The ozone layer is a consequence of oxygen-only chemistry.
Significant oxygen appeared on earth about 2 billion years ago due to rise of cyanobacteria.
cyanobacteria

5. Why it matters …
UV damages DNA

6. Structure of the Ozone Layer
note difference in profile due to absolute and relative concentrations. Absolute conc is more important in terms of ability to absorb UV light and in terms of reaction rates.
absolute conc profile peaks around km (lower-to-mid stratosphere)
typical VMRs for ozone in the stratosphere: 1 – 10 ppmv. Typical number densities: 1011 – 1012 /cc
questions: why does the ozone layer exist? Is it connected with the fact that there is a thermal inversion throughout the stratosphere? (Answer: dioxygen photodissociation essentially provides the heating mechanism for the stratosphere.
Observations: (i) O3 is not the most concentrated gas in the ozone layer (not even close!) (ii) maximum concentration is in the middle stratosphere.
Why does the ozone layer exist in the stratosphere?
What processes are responsible for its formation and maintenance?

7. Radiation and Matter
Also dependent upon the frequency of radiation!

8. Emission Spectra: Sun and Earth

9. INCOMING Solar Radiation budget
Incoming Solar energy…
Percentage
Reflected
35%
Absorbed by atm (ozone)
18%
Scattered to Earth from blue sky
10%
Scattered to Earth from clouds
15%
Radiation going directly to Earth surface
22%
100%

10. Composition of the atmosphere
Nitrogen: 78%
Oxygen: 21%
Argon: %
CO2: 380 ppm
Plus: CH4, H2O, He,

11. Atmospheric layers
60 km
15 km

12. Why is the Sky blue?
Ozone scatters short-wavelength (blue) incoming radiation from the sun.
Rayleigh scattering of incoming, short wavelength radiation (photons with specific energy)
Radiation scattered by O3, O2 in stratosphere (10-50 km)

13. Why are sunsets red? Blue wavelengths are scattered/absorbed
Red and orange pass through to surface

14. Blackbody emission curves and absorption bands

15. Photon Absorption: Vibration, Dissociation, and Ionization

16. Absorption of incoming solar radiation
1.The shortest wavelengths of the solar spectrum, the ultraviolet < .12 microns are absorbed by oxygen and nitrogen in the upper atmosphere above 100 kilometers.
2.UV radiation between between .12 and .18 microns are absorbed by oxygen above 50 kilometers.
3.UV radiation between .18 and .34 microns are absorbed between 50 kilometers and 10 kilometers by ozone.
4.The atmosphere is nearly transparent to visible radiation between .34 and .7 microns.
5.The infrared part of the solar spectrum is partially absorbed by water vapor in the troposphere below 10 kilometers.

17. Thermal Structure of the Atmosphere

18. Why are the Thermosphere and Stratosphere so “hot”?
Photodissociation of O2
O2 + hn  O + O ionization
l nm
Stratosphere:
Ozone production and destruction
is exothermic.
O2 + O  O3 (production)
O3 + O  2O2 (destruction)
produces heat
l < 310 nm

19. Photodissociation of Oxygen
O2 + UV light ( nm) --> O + O
Makes “free O” for making ozone (O3)
Occurs above 50 km in atmosphere
(Upper Stratosphere)

20. “Good” and “Bad” Ozone
Now

21. Natural Ozone Production
The Chapman Profile: balancing density and photon flux

22. Factors controlling the rate of photodissociation
1.The wavelength of light.
The wavelength must be short enough so the wave has sufficient energy to break the bond between the two atoms in the oxygen molecule. The most efficient wave lengths for photodissociation occur in the ultraviolet (0.15µm).
2.Variation of oxygen density.
As altitude increases oxygen density decreases (Chapman Profile). The higher the oxygen density the greater the likelihood of having an interaction between an oxygen molecule and a photon.
3.Variation of photon flux Photon flux decreases with decreasing altitude because of photon absorption by the atmosphere. Rate of photodissociation of oxygen is greatest at an altitude of about 100 km:

23. The Chapman Cycle
1930s, Sydney Chapman proposed a series of reactions to account for the ozone layer: the Chapman Cycle
The Chapman Cycle explains how the ozone layer is formed and maintained. Describe this process in some detail.
Four chemical reactions:
Initiation O2 + light  2O (120 – 210 nm)
Propagation (cycling)
O + O2 + M  O3 + M* (exothermic)
O3 + light  O2 + O (220 – 320 nm)
Termination O3 + O  2O2 (exothermic)

24. The Chapman Cycle Oxygen-only Chemistry
Give the four Chapman reactions:
O2 + hv  2O (< 242nm)
O + O2 + M  O3 + M* (generates heat)
O3 + hv  O2 + O(1D) (usually followed by rapid deactivation to 3P)
O + O3  2O2
Need to talk about the time frames of the reactions. In one cc, typical rates for the four reactions are:
106 molecules of O2 photodissociate per second
105 molecules of O3 react with O per second
“cycles” of Ox conversions occur per second
Thus, the typical “chain length” of the cycles is about In other words, the rate of interconversion among Ox species is (very roughly) one thousand times faster than the rate of Ox production or loss.
“odd-oxygen” species (Ox) are rapidly interconverted
Ox = O + O3

25. Ozone Production (>50km) Ozone Destruction (50-15 km)

26. O3 production & destruction

27. DU: Ozone measurement unit
UV A not blocked by Ozone ( µm), UV B partially ( ), UV C ( ) totally blocked by Ozone.
Dobson Sp[ectrometer measures UV C / UV A ratio
In 1985, Joe Farman of British AA survey measured dramatic loss in ozone..
Dobson Unit (100 DU = 1 mm O3 at STP)

28. Enter CFCs What are they?
Chlorofluorocarbons, organic compounds developed (by Dupont) in 1930s as safe, odorless substitutes for refrigerants, aerosol propellants, spray foam and insulation.
Example: CFC-12  CCl2F2

29. Rowland & Molina & Crutzen (1974)
Nobel Prize (1995)
Discovered that CFCs can last s of years in atmosphere.
CFCs susceptible to break down by UV
Predicted that CFCs will reduce ozone inventories.
Proof that this was occurring came in 1985.
Montreal Protocol 1987

30. Key ingredients to make an Ozone Hole:
Chlorine: supplied by manmade CFCs
Cold: Antarctic Polar Vortex
Seasons: Dark and Light seasons
Clouds: Polar Stratospheric Clouds
UV radiation: Springtime sunlight

31. CFCs: Chlorofluorocarbons
CFCs introduced 1950s
“Miracle compounds”: inert, cheap, many applications.
Uses:
Foam & Insulation
Propellants
Air conditioning
Electronics

32. CFC Compounds

33. Antarctic Polar Vortex
Large polar land mass
Ice covered, large temperature gradients
Circular airflow around Antarctica
APV effectively creates an atmospheric “fence” impeding air exchange with other regions
(this is arctic polar vortex)

34. Polar Stratospheric Clouds (PSCs)
Ice clouds during Austral winter (no light, very cold)
PSCs concentrate, activate Cl (as Cl- and CLO)
Ice crystals act as reaction sites for O3 destruction
CFCs accumulate in stratosphere

35. O3 loss by UV photolysis Austral spring: sunlight appears, UV
UV radiation splits off Cl atom from CFC molecule
Ozone destroying reactions:
Cl + O3 --> ClO + O2
ClO + O --> Cl + O2
(Cl is free to react with another O3 again)
Net: O3 + O --> O2 + O2

36. UV radiation and CFCs

37. CFC and O3

38. The Ozone Hole – Explained!
review again the sequence of events leading the the formation of the ozone hole

39. Discovery of the Ozone Hole British Antarctic Survey (Farman et al 1985)
Molina et al. (1974)
Farman et al. (1985)

40. Antarctic Ozone Hole in Dobson Units

41. Area of the Ozone Hole

42. The Ozone Hole right now
Antarctic Ozone Hole:
Sept. 12, 2012

43. The Ozone Hole in 2007
Blue = low ozone levels

44. Current Status (2012) Ozone Hole Area SH Dobson units

45. Future projections

49. Full recovery takes a long time (50 years)

50. The world avoided…
April, 70-90˚N

51. By 2020, an Arctic “ozone hole” is apparent

52. As over Antarctica, the ozone losses are extreme, and spread to mid-latitudes

53. A Brief History
June 28, 1974, Drs. Sherry Rowland and Mario Molina published the first study warning that CFCs could harm the ozone layer (Molina and Rowland, 1974).
They calculated that if CFC production continued to increase it would cause a global ozone loss of 30-50% loss by (current number is 70%!).
They warned that the loss of ozone would significantly increase the amount of UV-B light reaching the surface, increasing incidences of skin cancer.
Although no stratospheric ozone loss had been observed yet, CFCs should be banned.
At the time, the CFC industry was worth about $8 billion in the U.S., employed over 600,000 people directly, and 1.4 million people indirectly (Roan, 1989).

54. Tactics of the Ozone Hole Skeptics (1970s - 90s)
Launch a public relations campaign.
Predict dire economic consequences.
Find and pay a respected scientist to argue your point.
Elevate discredited scientific studies.
Emphasize scientific uncertainty.
“Cherry-pick” data to support your view.
Disparage and impugn specific scientists.
Compliance puts the nation at an economic disadvantage.
More research is needed before action should be taken.
Argue that it is less expensive to live with the effects.
In October of 1994, Rep. Tom Delay introduced a bill that would have
essentially repealed the entire section of the Clean Air Act that
deals with ozone depleting substances. Some time later,
Rep. John Doolittle (Republican from California) introduced a
somewhat less extreme measure that would have pushed back the
phase-out date from CFCs from 1996 to the year (A friend of
mine remarked that "Doolittle and Delay" sounded like a pair of
characters from a Gilbert and Sullivan production.) Either measure
would have amounted to a US withdrawal from the Montreal Protocol
and its successors. After the Republicans took control of Congress
in the 1994 elections, the House Subcommittee on Energy and Environment,
now led by Rep. Dana Rohrabacher, decided to hold a hearing related to
these proposals. At the first panel, dealinf with the scientific issues,
Singer and Baliunas represented the skeptics, while
Robert T. Watson (then at the Office of Science and Technology Policy)
and Dan Albritton, director of the NOAA Aeronomy Lab, represented the
conventional scientific wisdom. (A fifth scientist, Richard Setlow
from Brookhaven, also spoke, but he dealt only with the biological
effects of UV radiation, not with whether ozone depletion was actually
taking place or not. I'm not sure why the Republicans brought him in.
A subsequent panel dealt with economic and policy issues.)
The hearings were held on September 20, 1995.
A great link to Jeffrey Masters’ article on this